Radiotherapy dosimetry audits

Expert audits and advice

At NPL, we offer a wide range of radiotherapy dosimetry audits, from reference dosimetry audits to end-to-end audit of advanced radiotherapy techniques, such as rotational intensity modulated radiotherapy (VMAT), stereotactic ablative body radiotherapy (SABR) and stereotactic radiosurgery (SRS).These audits are run in collaboration with the clinical trials QA group (RTTQA) and the UK SBAR consortium, where appropriate.

Reference dosimetry audits

Therapy level MV photons, electrons and medium energy X-rays audits based on the UK IPEM codes of practice are available. An audit at a single beam quality consists of a beam quality check, output measurement and field instrument calibration. Local paperwork, data recording by the centre and procedures are NOT examined. An on-site audit in a single modality takes several hours, depending on how many qualities are examined. The audits are arranged at mutually agreed times and can be performed after normal working hours, if required. Audit using the postal alanine service is also available. Alanine is a solid state dosimeter with a wide dose range (5-100kGy), low energy dependence and near tissue equivalence. The alanine pellets provided are traceable to the NPL UK primary standard, independently of the ion chamber chain.

Stereotactic ablative body radiotherapy lung dosimetry audit

NPL, in collaboration with Clatterbridge Cancer Centre, provide a postal audit of lung cancer stereotactic ablative body radiotherapy (SABR), following the protocol developed as part of the UK SABR Consortium Lung national dosimetry audit. The audit is designed to assess the positional and dosimetric accuracy of a SABR lung treatment delivery. This is achieved with the use of alanine pellets and EBT3 GafChromic film dosimetry, placed in a CIRS Thorax phantom. The CIRS thorax phantom is a segmented phantom with regions of lung, spine and water equivalent density.

The alanine pellets are housed inside a Farmer shaped holder, which can be inserted into the CIRS thorax phantom. This means that you can perform a position check measurement using your own Farmer ionisation chamber prior to the audit measurements to ensure accurate setup.

In this audit the alanine and Gafchromic film are mailed to your centre. Each centre either uses their own CIRS Thorax phantom or borrows one from a nearby centre, we are happy to help put you in touch with another centre if required.

This is a remote end-to-end dosimetry audit of intracranial stereotactic radiosurgery (SRS). The audit tests the dosimetric accuracy of all aspects of the SRS treatment pathway, which are immobilisation, CT scanning, import of images into the Treatment Planning System (TPS), image fusion and treatment delivery. It is advised to include the multi-disciplinary team in the relevant steps in this audit. The audit uses the same protocol as used during the NHS England national SRS dosimetry audit. Therefore there is an added advantage that your results will be benchmarked against over 30 centres results.

The phantom used in the audit is a bespoke CIRS STEEV which has been adapted so that the alanine and Gafchromic EBT-XD film measurements can be performed simultaneously.

The STEEV phantom is delivered to your centre pre-loaded with alanine and EBT-XD Gafchromic film, where you then send the phantom through your SRS treatment pathway from CT scanning through to delivery of the dose. Afterwards the phantom is returned to NPL where the alanine and Gafchromic films are unloaded and analysed.

As part of this audit we also include a reference photon output measurement using alanine housed in a Farmer shaped insert, which can be inserted into your own solid water blocks.

Rotational IMRT audit (VMAT and Tomotherapy)

We can offer an on-site independent audit of rational intensity modulated radiotherapy (IMRT) techniques, VMAT and Tomotherapy. This audit makes use of either the PTW Octavius II Octagonal phantom plus the 729 Detector Array or the Octavius 4D phantom plus the 1500 Detector Array. The former follows the protocol used during the 2013 National VMAT and Tomotherapy audit. Each centre creates a plan on the 3DTPS test developed by RTTQA, and measurements can also be performed using centres own clinical plan.

Bespoke audits

We are also able to use our existing expertise to discuss and support bespoke audit needs. For example, we have previously provided alanine pellets for independent measurement of VMAT treatment plans. The centre is able to use their own phantom or borrow a phantom and make measurements on specific plans of their choice. NPL will report the measured doses which can then be compared with the locally expected doses.

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